Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations cover the safety profile (read-only, idempotent, non-destructive, closed-world), and the description adds behavior they cannot: ambiguous place names return candidates rather than a guess, end defaults to start making a round trip, departure defaults to now, and a bare '08:00' is interpreted as today. It also notes each leg carries a picture_url. That is meaningful operational context beyond the structured fields.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with purpose, then routing, then input semantics, then an example — a sensible order with no filler sentences. It is on the long side, and the extensive start-format enumeration overlaps schema documentation, but each block carries usable information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return-value explanation is not required, and the description still notes the per-leg picture_url. Combined with defaults, disambiguation behavior, and the worked JSON example, an agent has everything needed to construct a correct call.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3, but the description goes further than the schema on several parameters: it enumerates the accepted forms for start (station name, address, named place, 'geo:lat,lon', place_id from find_place), restates the 1–12 stop limit, and clarifies leave_at's night-away semantics. It omits transfer_time_factor, which only the schema documents, so it does not fully compensate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource ('Plan a whole day of public transport with time spent at each stop') and unpacks what a day plan contains — outbound train, stay, connection, return — which is a distinct capability from a point-to-point planner. It explicitly contrasts itself with plain journey planners and with the sibling plan_journey, so an agent can separate the two without opening a schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives concrete use cases (multi-stop day trip, three towns in a day, evening out and back) and an explicit exclusion: 'For a single A-to-B trip, use plan_journey.' The when-to-use and when-not-to-use conditions are both stated outright.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.